Single-sided stretch cling films are used by the packaging industry to over-wrap packages of goods, particularly they have been used to securely hold and/or wrap an article or group of articles, such as the use of an over-wrap film for the unitization of a pallet full of goods. In these applications, the films ideally should have high strength, be resistant to tear and puncture, and exhibit single-sided cling properties. The single-sided cling properties are important to prevent individual pallets' over-wrap from clinging to each other, which could result in tearing and/or destruction of the over-wrap and, ultimately, separation of goods from the pallet.
Generally, single-sided cling film is comprised of a cling layer and a release layer. The release layer does not have to have non-cling characteristics, but it does typically exhibit lower cling characteristics than the cling layer. The cling film is typically co-extruded with an A/B film layer structure. The cling layer typically comprises the A layer while the release layer typically comprises the B layer. In general, the cling force measured between two A layers of adjacent film structures and between the A layer of one film and the B layer of another film, is much greater than the cling force exhibited between the B layer of one film and the B layer of an adjacent film. Sometimes a third layer, C, is added to form an A/C/B film structure. In this type of film, the B is again the release layer and the C layer is a core layer that is used to obtain specific properties in the cling film and can consist of more than one physical layer. For example, if a higher modulus film is desirable, then the C layer may comprise a polymer that exhibits a higher modulus compared to the A or B layers.
Typically, the single-sided stretch cling film is wrapped around the goods to be unitized (such as cases of goods loaded on or to be loaded on a pallet), either using a hand applicator or a semi- or totally automated machine, in such a manner that a cling layer is laying over and adjacent to release layer of film. In most typical applications, the film needs to be stretched as it is applied to the goods in order to develop sufficient holding force for maintaining the integrity of the goods (such as a pallet of goods) to be unitized. This typically requires the single-sided stretch cling film to be capable of being stretched to 100%, often 200%, of its original length, while still maintaining its mechanical integrity and exhibiting adequate cling force between the cling layers and release layers where they contact.
In order to provide film structures providing adequate holding force, ethylene homopolymers and ethylene-based copolymers have been utilized (both hereinafter sometimes referred to as “polyethylene(s)). Polyethylene, having densities greater than 0.915 g/cc, provide adequate holding force, but they do not provide sufficient cling force for most applications. Therefore, the single-sided stretch cling films utilizing lower density polyethylene for the cling layer and higher density for the release layer have been developed. Unfortunately, for a film of a given overall thickness, a film using a lower density polyethylene in the cling layer and a higher density polyethylene in the release layer often has substantially lower holding force than a film made of just the higher density polyethylene. This will require a cling film having a larger thickness than desirable. Additionally, very low density polyethylene (having densities between 0.880 g/cc and 0.914 g/cc) are typically necessary to obtain the desired cling properties.
Single-sided stretch cling resins utilizing a polyethylene of sufficiently low density to provide adequate cling properties, often generate considerable noise as they are unwound from their supply roll in the process of wrapping the goods. In some cases, such as where extremely low density polyethylene is utilized, the noise generated may be in excess of that which can be safely tolerated for long-term exposure by the human ear. This is particularly problematic when the film is to be unwound by hand or using a semi-automated machine, because of the presence of personnel close to the supply roll.
As an alternative to films using very low density polyethylene for the cling layer, single-sided stretch cling films have been developed which utilize minor amounts of low molecular weight tackifiers. Common tackifiers include polybutylenes, mono- and diglycerides of fatty acids, low molecular weight amorphous polypropylenes, terpene resins and rosin esters.
These tackifiers tend to be migratory within the film and can cause die-lip build-up during film fabrication, and other undesirable accumulations on equipment during wrapping operations, that necessitate periodic stoppage of operations for cleaning and maintenance. Moreover, since they are migratory, they often migrate to adjacent and other films layers (and into the release layer of an adjacent film), thereby causing undesired adhesion and dirt retention problems commonly associated with two-sided cling film structures. Also, if the tackifiers migrate to the release layer of the film, it may result in one unitized pallet of goods to cling or drag against an adjacent unitized pallet thereby causing transportation and handling problems. Because of the cling nature imparted to the over-wrap by the migrated tackifier, one such over-wrapped pallet of goods may not readily slide against an adjacent pallet of goods. The tendency is for the over-wrap of one pallet to pull destructively upon the over-wrap of an adjacent pallet because of the cling nature imparted by migrated tackifier. This can be a problem in wholesale and retail goods distribution centers where pallets of goods are often moved about by fork lifts which can impart enough force to pull through the stretch over-wrap and destroy the integrity of the unitized pallet of goods.
Further, the tackifiers tend to be expensive and add to the cost of the overall film structure. The migratory tackifiers tend to be difficult to use for single-sided stretch cling film structures. Of particular concern, the migration of the tackifiers is subject to the duration of time since the film was made, the temperature under which the film was made and stored, the molecular characteristics (such as the molecular weight) of the base resin to which they are added, and the processing conditions utilized for over-wrapping the goods, such as winding tension. These result in films exhibiting widely divergent cling properties based on the properties present during their manufacture, storage and use. It also makes it difficult to use a single tackifier to manufacture a variety of different single-sided stretch cling films, of which each are designed to utilize different base polyethylene resin for the cling layer. Since the tackifiers are typically low molecular weight and amorphous, they can be difficult to handle in the context of typical thermoplastic film forming operations and can require specialized handling equipment, which adds to the complexity needs for manufacturing the single-sided stretch cling film. An additional layer can be provided between the cling and the release layer that comprises a thermoplastic resin that acts as a barrier to the migration of the tackifier. However, this additional layer further adds to the cost and complexity of manufacturing the film.
Single-sided stretch cling films have been disclosed wherein the cling layer comprises functionalized polymers, such as ethylene acrylate and/or ethylene vinyl acetate copolymers. While the functionalized polymers tend to improve the cling properties of the film, they also tend to be incompatible with the base resin making up the majority of the cling layer. This incompatibility can result in rheology matching problems in co-extrusions with polyethylene as well as thermal instability. Additionally, these functionalized polymers can lead to incompatibility problems during recycling of edge trim and film scrap generated during the film fabrication process. Further, the functionalized polymers are expensive and can greatly increase the cost for manufacturing the films.
What is desired is a composition that does not need to contain a migratory tackifier or functionalized polymers but, when incorporated into the cling layer of a single-sided stretch cling film, results in film that provides adequate cling properties, but also exhibits acceptably low noise when the film is unwound from film supply roll during over-wrapping operations. Additionally, it is desirable that the composition comprise at least 80 percent by weight of a polyethylene having a density of at least 0.905 g/cc. Further, it is desirable that the composition contains a non-migratory cling additive that will be useful for providing acceptable cling properties across a wide variety of different polyethylene base resins.